SINGLE-BUNCH TUNE AND BEAM SIZE MEASUREMENTS ALONG BUNCH TRAINS IN PEP-II*

By scanning gated cameras and gated tune monitors across the bunch pattern during normal colliding-bunch operation of PEP-II, the tunes and beam sizes of individual bunches were measured simultaneously in the high and low energy storage rings of PEP-II. The measurements were made with 1561 colliding bunches in PEP-II, arranged in trains of 66 bunches, with each bunch in the train separated by 4.2 ns. The tune and beam size measurements were correlated with the current, luminosity, and specific luminosity of the bunch. The results show a vertical tune shift at the start and end of the mini-trains, a luminosity droop along the mini-train, and specific luminosity drop in the first and last bunches of the train, since they experience a different parasitic crossing on either side of the interaction point (IP). PEP-II OPERATING CONDITIONS PEP-II consists of two storage rings, a high-energy ring (HER) with 9-GeV electrons, and a low-energy ring (LER) with 3.1-GeV positrons. The highest luminosity, 9.2x1033 cm-2 sec-1, was obtained by colliding 1588 bunches of 2.45A in the LER with 1.55A in the HER. This high luminosity requires filling every second RF bucket, resulting in a small transverse separation—called a parasitic crossing—between bunches both entering and exiting the IP. The parasitic kicks the passing bunches give each other can shift the tune and lower the beam lifetime. If the beam is Gaussian transversely, the parasitic tune shift is given by [1